Embossed cold rolled metal (e.g. steel) sheets are commonly used in many applications, one such use being for appliance (e.g. microwave, refrigerator, etc.) panels. FIGS. 1 and 2 show a typical prior art embossed sheet of cold rolled steel, with FIG. 1 being a top photographic view of the unpainted sheet and FIG. 2 being a side enlarged cross sectional view of the sheet with paint applied thereto. The cold rolled sheet includes steel substrate 3 covered with a layer of paint 5. In the embossing process, a plurality of impressions 7 are formed in steel substrate 3 by working rolls (not shown). Plateaus 9 are formed between impressions 7.
In the manufacture of cold rolled steel sheets, it is often desirable to control the surface finish (i.e. embossing) of substrate 3 in order to enhance the appearance and performance of the sheet in subsequent operations. Embossing is commonly done in the final stages of manufacture by rolling the strip or sheet 3 between a pair of opposing working rolls, one of which is textured of predetermined configuration so as to impress the embossing pattern into a surface of sheet 3. See, for example, U.S. Pat. Nos. 4,059,000 and 4,092,842.
Surface texturing of the working roll is an established technology in embossing steel sheets and in the printing industry. The protrusions (i.e. surface texture) defined on the working roll are often created via acid etching. The surface texture of the working roll is transferred to the cold rolled sheet during the embossing process, thereby forming the pattern.
Unfortunately, prior art embossed cold rolled steel sheets, such as the one illustrated in FIGS. 1-2, suffer at least from the following problems: (i) non-uniform paint coverage of the surface; (ii) high paint consumption; (iii) premature corrosion (e.g. rust) due to paint thinning at the transitions 11 from plateaus 9 to impressions 7 as shown in FIG. 2; (iv) discoloration when light paint colors are used due to the dark base metal showing through the thin paint at transition points 11; (v) loss of the embossed pattern after painting; and (vi) not allowing for re-painting without negatively impacting the aesthetic appearance of the sheet.
The inclination (i.e. slope) of surfaces 13 of substrate 3 between plateaus 9 and impressions 7 has been found by the instant inventors to be an important surface feature with respect to premature corrosion, discoloration, and non-aesthetically pleasing appearance. Unduly high slopes of surfaces 13 cause paint 5 to flow downward into impressions 7, leading to paint thinning at transition points 11 between plateaus 9 and impression 7 (see FIG. 2), which makes these sites prone to premature corrosion. Additionally, when light colored paint is used, thinner paint at transition points 11 leads to non-uniform appearance due to the dark base metal of substrate 3 showing through the thin paint at these points. Slopes which are too low, on the other hand, cause a loss of crispness in the painted embossed surface, which leads to a less aesthetically pleasing appearance.
The depth of impressions 7 relative to the tops of plateaus 9 has also been found by the instant inventors to be an important surface characteristic with respect to paint consumption, allowance of re-painting, and paint thinning across the plateaus. Impressions 7 which are too deep create deep pockets for high paint consumption, and/or excessive paint flow from the tops of plateaus 9 into impressions 7. This creates thinner paint coverage across plateaus 9 and transition points 11. Impressions 7 which are too shallow, on the other hand, cause loss of the embossed pattern after painting, and prevent repainting by negatively affecting the appearance of the painted surface.
Surface roughness of plateaus 9 is another characteristic which has been found by the instant inventors to be important with respect to appearance and paint adhesion problems. Average surface roughness values (Ra) on plateaus 9 which are too low cause paint 5 to flow rather easily down and across surfaces 13 into impressions 7 thus leading to thin paint coverage across plateaus 9 and transition points 11. This gives rise to paint adhesion and corrosion problems. On the other hand, average roughness values which are too high, although favorable from a paint flow point of view, lead to large variations in the appearance of the painted cold-rolled embossed steel sheet. Average surface roughness (Ra) is discussed, for example, in U.S. Pat. Nos. 3,754,873 and 5,182,171.
The sharp corners or ridges at points 11 shown in prior art FIG. 2 lead to undesirably thin paint coverage at these points, thereby rendering them susceptible to premature corrosion.
In sum, prior art cold rolled embossed steel sheets suffer from the above referenced problems. In view of this, there exists a long felt need in the art for a paintable cold rolled embossed steel sheet and method of making same whereby the sheets are manufactured in order to achieve (i) uniform paint coverage across all surfaces; (ii) acceptable paint consumption; (iii) no premature corrosion due to paint thinning at transition points; (iv) improved aesthetic and color appearance; and (v) capability to be repainted without negatively impacting the appearance of the embossed sheet.